This invention relates to combination power plants in which a steam power generation plant and an ocean thermal energy conversion power plant are combined, and more particularly to a combination power plant which avoids or minimizes environmental problems associated with warm water which is discharged from the condenser in the steam power generation plant.
Generally, thermal power stations or nuclear power stations (which are referred to hereinafter as "steam power generation plants"), use steam produced in a boiler or in a nuclear reactor. This steam is used to rotate a turbine-generator and is then condensed in a condenser. A large quantity of water is required to condense the steam, so steam power generation plants usually are built along a river, lake or sea. One of the problems which arises is that warm water which is discharged from the condenser back into the river, lake or sea can cause environmental disruption. Therefore, it is important to lower the temperature of the water discharged from the condenser to minimize the environmental disruption effect on the body of water into which it is discharged. It is also important to minimize the quantity of water which is required for heat exchange in the condenser.
It is difficult to maintain a satisfactory level of thermal efficiency of a steam power generation plant when both the quantity of water used for heat exchange and the discharged water temperature is lowered. Also, it is difficult to lower the temperature of the water discharged from the condenser, because it is necessary to use a large quantity of cooling water in order to lower the temperature of the discharged water.
Another problem is that the temperature of the seawater (or river or lake) which is used as the cooling water for the steam power generation plant varies in different seasons. In summer the temperature of the seawater rises causing a corresponding pressure rise in the condenser. Accordingly, a greater quantity of steam is required for the turbine to maintain the rated output of the generator which is driven by the turbine. The steam turbine and the condenser generally are designed to provide the rated output at the maximum seawater temperature.
It will be appreciated that the seawater temperature is lower in the winter as well as in the spring and autumn, than it is in summer. Accordingly, the steam turbine and the condenser have excess capacity which is not utilized in the cooler seasons and which adds to the construction cost and equipment investment and lowers the plant efficiency. Designing other than to the maximum seawater temperature will result in less than the rated output of the power station at high seawater temperatures.
Ocean thermal energy conversion power plants using working fluids such as ammonia or Freon are similar principle to the aforementioned case. Ocean thermal energy conversion power plants use the temperature difference between the relatively warm seawater from the ocean surface and the cool seawater from the bottom of the ocean (or at a substantial depth). The working fluid such as Freon or ammonia is evaporated in an evaporator by surface seawater at about 30.degree. C. introduced into the evaporator. The vaporized Freon rotates a binary turbine connected to a generator. The working fluid vapor exhausted from the binary turbine is introduced into a condenser where it is condensed by seawater from the ocean floor at about 7.degree. C. The condensed working fluid is supplied to the evaporator by a working fluid pump and is evaporated again and then supplied to the binary turbine.
The temperature of seawater at a depth of about 500 to 600 meters is constant throughout the year whereas the seawater temperature at the surface varies considerably and is lower in winter. If the temperature difference between the warm surface seawater and the cool water at the ocean floor is not sufficient, e.g., 20.degree. C., plant efficiency is reduced. This can occur particularly in winter.